1. Field of the Invention
The present invention relates to communication terminals and card antenna modules, and particularly to a card antenna module preferably for use in an RFID (Radio Frequency Identification) system and a communication terminal including the same.
2. Description of the Related Art
RFID systems serving as article management systems are known. In RFID systems, a reader/writer and an RFID tag communicate with each other in a non-contact manner, and information is transmitted between the reader/writer and the RFID tag.
Each of the reader/writer and the RFID tag includes an RFIC chip for processing radio signals and an antenna for transmitting and receiving radio signals. Certain information is transmitted and received between the antenna of the RFID tag and the antenna of the reader/writer via a magnetic field or an electromagnetic field.
In recent years, there have been some cases where an RFID system is introduced to a communication terminal such as a mobile phone, and the communication terminal is used as a reader/writer or an RFID tag. As a technique of giving an RFID function to a communication terminal, a technique of using a card antenna module is known, as described in, for example, Japanese Unexamined Patent Application Publication No. 2006-054772 and International Publication No. WO 2007/125948. With this technique, a card antenna module including an antenna and an RFIC chip is inserted into a casing of a communication terminal, and the communication terminal provided with the card antenna module is used as an RFID reader or an RFID tag.
In recent years, casings of communication terminals often include an antenna for an HF-band RFID system, such as FeliCa (registered trademark) and NFC (Near Field Communication). When a casing of a communication terminal includes an antenna for an HF band, particularly when a card antenna module includes an antenna for an HF-band RFID system, the module-side antenna and the terminal-casing-side antenna are coupled with each other via a magnetic field, and the module-side antenna may not operate properly.
In contrast, if a magnetic or metallic flat plate is attached to a card antenna module so that a terminal-casing-side antenna is shielded by the magnetic or metallic flat plate when the module is inserted into the terminal casing, coupling between the module-side antenna and the terminal-casing-side antenna can be reduced. However, if the terminal-casing-side antenna is not completely covered, coupling between the module-side antenna and the terminal-casing-side antenna occurs. Also, a magnetic flat plate or a metallic flat plate having a large area needs to be provided to completely cover the terminal-casing-side antenna. That is, the card antenna module is operated by inserting it into a slot of the terminal casing, but it is difficult to completely shield the terminal-casing-side antenna only by inserting the module into the slot.